Presently, there are several methods for indicating an impending aircraft stall situation. One method utilizes a computed stall warning airspeed which is identified by a symbol at a corresponding location on an airspeed indicator.
Another method of indicating a stall condition of an aircraft is the utilization of an angle of attack indicator which can be located in the aircraft cockpit, and which provides a direct indication of the aircraft flight situation in relation to a stall condition. An angle of attack indicator uses an indicating scale which may be scaled in arbitrary units. A pivotal vane typically located on the outside of the aircraft senses the current angle of attack of the airflow along the aircraft, and feeds the information, after proper scaling, to an angle of attack indicator. The stall angle of attack is sometimes indicated by a symbol on the angle of attack scale.
Warning devices such as stick shaker and stallhorns are also utilized to warn of an impending stall condition. These devices are typically actuated prior to the aircraft entering stall buffet in order to give the aircrew advanced warning of an impending stall condition. In the case of a stick shaker, when the aircraft first reaches the predetermined stall warning limit, a motor is activated which causes the aircraft control stick to begin vibrating rapidly providing the aircrew with a tactile warning of an impending stall condition.
No indications are currently known to exist which allow the pilot to effectively accomplish a pitch up maneuver without exceeding the maximum allowable angle of attack. Current stall warning devices, such as stick shakers, present no information during a pitch up maneuver until the selected angle of attack threshold is exceeded. Angle of attack indicators have technical potential in this regard, but are not operationally practical for use in a marginal performance situation near the ground. This is because an angle of attack indication is rarely used in commercial transport airplanes during normal operations. Its use by the pilot would therefore be unlikely in the rare situation when a maximum lift capability maneuver is required. To fly out of such high stress situations, the pilot will normally rely on instruments that he is accustomed to using in routine situations.
Displaying a computed stall warning airspeed, or any other airspeed based on stall, on the airspeed instrument is another means by which the maximum allowable lift capability could theoretically be used by the pilot. However, this concept has the following disadvantages, namely (1) stall speed varies as the square of load factor which causes stall speed to change rapidly during pitch maneuvers making it a difficult parameter to control or to avoid, and (2) a pilot cannot control speed directly; only acceleration and deceleration can be controlled directly by varying pitch attitude and thrust.
A stall warning device is disclosed in U.S. Pat. No. 3,686,936--by Daudt, Jr. The instrument includes a first scale calibrated in degrees of angle of attack together with corresponding pointer which points to a location on the scale to indicate the current angle of attack of the aircraft. A second Mach number scale located adjacent to the angle of attack scale indicates the Mach number corresponding to the angle of attack at which stall buffet will begin.
In Lear, U.S. Pat. No. 2,942,233, there is disclosed an instrument for indicating the pitch attitude of an aircraft, and the relationship of aircraft position to a radio beam defining a selected glide slope.
In U.S. Pat. No. Re. 31,159 by Sicre, there is disclosed an aircraft instrument for guiding the aircrew during takeoff and landing approach go-around. The instrument includes a horizontal bar which is displaced relative to an aircraft symbol at a distance which is a function of a generated error signal representative of the difference between an aerodynamic flight path angle and a desired flight path angle.